Charged-particle energy meter



Patented Dec. 4, 1951 UNITED PATENT OFFICE CHARGED-PARTICLE ENERGY METER John H. Coleman, Princeton, N. J., assignor to Radio Corporation ofgAmelfica, a corporation Applicationnpril i, 1949, SerialNo. 84,785

This invention relates generally to" ch argei'lparticle energy meters and more, particularly, to

such meters in which the energy of the radiated particle is determined as a. function of ;the rat'i,o

of the secondaryelectron emission to the'primary radiation, following the particlesbombarding a target.

1 arolaimst-f (o1. 25083.6)-

It is known that when a surface is bombarded by radiated nuclear particles, secondary electrons areemitted intopspacejfrom the surface. Even materials that are classed generally as insulators are subject to some secondary electron emission upon bombardment-bych'arged particles The number of secondary electrons emitted from a surface per individual bombarding par-' ticle, which defines the secondary emission ratio dsvaries for "an individual-material "with" the energy oftheinoident particles. Also; the secondary emission ratio varies among difl'erentmate rials for the same incident energy. '1 And, "the" variations in the secondaryemission ratio 'forflow ener y particles is inherently different-from the variations forhigh energy particles} for the sa e material. H

The Present invention, provides a-nove'l method of and apparatus for determin'in'g'the secondary electron emission ratio when a targetof known material is bombarded by a'beam of'ch arged par ticle radiation of l known energy, from which may be determined'the relation of the secondary emission ratio to the energy of the particles-for var-'' ious target materials and the relation of the se c ondary emission ratio to theatomicweightof the material of' which the target ismade,=forvarious particle energy value 'lhese data in'the form of curves are then available for use asa-step in the method of determining the unknown energy values of a charged particle radiation,th'at is, of determining the energyof the radiationl'particles inelectron volts. I V

The principalobjec't of the invention is 'to'pro vide a novel method of andapparatus for determining the energyof the particles in a charged particle radiation.-

Another object of the invention is to provide a novel method of and apparatusfor determining-the secondary emission ratio ofa "target bombarded by nuclear charged particles.

Another object of the invention is to provide a novel method'of and apparatusfordetermining the secondary emissionratio of a target bom barded by primary charged particles and from the determined ratios, determine the energy of the unknown primaryparticlefradiation.

'Another object of the invention is to-provi'de-a Figure 2 illustrateszby curves "typical relations of thesecondary emission ratios to the various bombarding energies for severaltarget materials; Figure 3 is a schematic sketch ofa face View of the-same general arrangement as shown in Figure 1, except that the target is composed of two semicircular discs of 'differentmaterials and with separate recording circuits for each target;

Figure 4 is agraph'showing the-relation of the secondary emission ratios to the energies of the primary-particles-"for two materials being bombarded at relatively low energy values; I

Figure 5 is a schematicsketch of a face view-of the same general arrangement as shown in,Fig-I-.

ure 1, except that the target-iscomposedof relatively narrow strips of two different materials, forming in general a circular disc target, the'alternatestrips being connected together and to exteriorcircuits; s

Figure 6- is a schematic sketch-of a side view of a modification of device in'Figurel'in which the collector is spherical and transparentto high energy particles;

Figure- 7 is a schematic sketch "of a'further modification of the device shown inFigure l, in which the collector is shown as a truncated ho1- low cone;

Figure 8 is a schematic sketch of a multikinetic beam of charged particles being separated by a magnetic-field and showing the position of the collector relative to the separatedbeam; and Figure 9 illustrates the relation by'a curve of secondary emission ratios to the "atomic weight of the target material for a constant energy of the bombarding particles.

Similar reference characters are applied to similar elements throughout the drawings.

Referring to Figure l, the arrows, numeral i0, represent twoof thefpaths'of charged particle radiation, "for example, electrons bombarding In describing the basic embodiment target I I.

of the invention, the radiation will be assumed to be mono-kinetic. This primary radiation causes secondary electrons to be emitted within the evacuated envelope 9 from dynode target I I, the paths of four of which are shown as arrows I2. The electrons leave target II at various velocities, from nearly zero to approximately the velocity of the primary particles, and are collected by the flat ring like collector I3, which is biased positive by being connected to electric source I4, the negative side of which source is connected to ground I through ammeters I6 and I1 and to target II through ammeters I6 and I8. Ammeter I5 is connected between collector I3 and ammeter I! to measure the secondary emission current ids. The currents flowing to and from In operation: Having determined by observation the values of ipa minus isa and ipb minus isb, collector I3 is biased sufiiciently negative to suppress secondary currents isa and isb, which is determined by increasing the bias on collector I3 until ammeter I6 reads zero. The values of ipa and ipb are then observed by reading ammeters I 8a, and l8b, respectively. The differences, then, between the secondary emission ratios of the two targets, dsa minus dsb, may be determined by isa isb captured primary electrons bombarding the target II, forming primary current ip, and the secondary electrons emitted from the target I I and collected by collector I3, forming current ids. The net current flowing to target II is ip minus ids. The current flowing through ammeter I! to point I9 is equal to the sum of the currents flowing" through ammeters I6 and I8 away from point I9, or in minus ids plus ids, or ip.

There is thus provided circuits and instruments to measure the primary current ip and secondary current ids. By definition, the secondary emission ratio is equal to ids divided by ip.

These secondary emission ratio data constitute the characteristics of the target under the observed conditions. The data may be assembled in the form of tables orcurves or families of curves and thus these data become available to determine the unknown energy of the particles of the radiation when the secondary emission ratio of that radiation is observed from targets of known materials. 7

The family of curves in Figure 2are determined by exposing targets to known energy radiations, the targets being made of various elements, such as tungsten or graphite, or compounds of elements, such as steel or beryllium oxide, and plotting the relations of the secondary emission ratio to the energies of the-particles.

The energy of an unknown radiation may then be determined by entering the curve, as shown in Figure 2, by drawing a horizontal 'line on the figure corresponding to the determine value of ds and picking off the corresponding value of energy of the primary electrons on the curve of the material of target II.

Referring to Figure 3, there is illustrated therein a modification of the device shown in Figure 1, in which the target is composed of two semicircular dynode discs IIaand Nb, made of different materials, for exam le, beryllium and! and carbon, respectively. These targets are separated in space and are connected, through ammeters l8a, and I 8b, respectively, to point I9.' I3 collects the secondary electrons meter I8b will register the currents ipb minus isb and ammeter I I will register the primary currents ipa plus ipb; J

The observed data and the calculated value of dsa minusdsb are used to determine the energy of the bombarding particles by applying the dif- ,ference to curves plotting the relation of ds to particle energy for the two materials of the target, IIa and III), such as shown in Figure.2. There is enough divergence in the curves for the various materials to determine the particle en-' ergy. For example, if target section Ila is made ratio and the energy of the bombarding particles varies greatly for the different materials of the target for the lower energy values. The curves for these values of bombarding energy are of the same general shape, having a rise to a peak and a decline at a less rapid rate for further increases in bombarding energies.

Advantage may be taken of the characteristic curves of different materials, such as shown in Figure 4, in which curve 2!! is a plot of (is versus ev for beryllium oxide on a beryllium dynode and curve 2I is a similar plot for graphite. Not only do these curves vary greatly between themselves, due principally to variations in particle capture, but the curves vary greatly in their slope, thus increasing the accuracy of fitting the values of dsa minus-dsb to the curves to pick off from the curves the value of ev.

In the determining of the value of co in the example shown in Figure 4, the value ofalso. minus dsb may fit between the curves in more than one value of ev. The value of en is generally known within general limits so that the proper position may be selected, with aid in selectionby knowing whether the values of dsa minus dsb is decreasing or increasing, and applying the value according to the relative slopes of the curves.

When it is desired to determine the energy values of an inhomogeneous energy radiation, the rays are passed through a magnetic field, shown generally, at 22, Figure 8. The rays are directed through the field at right angles to the magnetic lines of force and their. paths are bent inversely as their energy values. By making the magnetic field of sufficient value, the beam is s read out to such an extent, that the rays of the beam are mono-kinetic at various positions around an are relative to the point where the beam enters the magnetic field. The target II may be moved progressively into the paths of rays Illa, lIlb,

5,, Mo etc., -and-thewnergy-valuesof ':the: component parts of the beam may be determined Other embodiments of the invention are obvious, for exampla as' disclosed in FiguIes 5, 6 and 7.

With referenceto Figure; 5, the two sections of thetarget, lla and lib, made of different materials as hereinbeiore disclosed, are divided-upv intostrips, narrow in proportion to the diameter of the beam. These .stripsof two dilferent materials are mounted alternately across thebeam and are connected together as shown at point I9 through ammeters IBaand 18b, respectively. 001-.

lector l3,is biased byelectric source Mandis connected. to point l9 through ammeter l 6; Point I9 is grounded vat l5 through ammeterll. .l

' Thesecondary emission electrons. are collected generally uniformally over the beam area, even though target II a is of difierent materialfrom target llb. The primary current, indicated by ammeter I1, is, therefore, equally divided between the two target circuits. The current flowing through'amm'eter [8a. ip/2 minus isa; the current flowing through ammeter l8b is iii/2 minus ,isb; and the current flowing through ammeter i6 is isa plus isb. j

By definition dsequals the secondary current divided by the primary current. Therefore,

, isa j sb barding target H, but thick enough to collect secondary electrons I 2. The remainder 23 of the envelope 9 is madeof some insulating material and the envelope is evacuated.- Connections toand from electric source l4, ammeters [6,41 and i8, and ground [Scare-the same as disclosed in connectionwith Figured. I

In Figure"? is shown another embodiment of the invention wherein the collector Isis in the form of a truncated cone. made of. sheet material. The evacuated envelope 9 consists of a thin window 24 and a section 23, as described in connection with Figured By making the envelope out of: a material that wouldabsorb: radiation with-,

out secondary emission. or by grounding section 2.3, the section, 23 will ,shield collector Ll3 from bombardment by stray rays infthe radiation beam.

It should be understoodthat either an externally or internally disposedelectron multiplier arrangement may be combined with any of the embodiments of the. invention described heretofore to provide any, desiredldegree'ofcurrent amplificationof the secondary electrons released from the radioactive target ll.

Data is" also obtainable, as hereinbefore -'disclosed; to plot the-relationof the secondary emission ratio ds versus atomic weights of elements of the target: for: a constant energy- 0f particle bombardment; I

Figure 9 is a typical' curve of. ds versus atomic weights of the target fora constant energy value of 300;00'0 ev. A family of curves foriother constant values offthe energies of the bombarding particles may-be plotted. HaVll'lg determined by a device such as shown in'Figures 1, 6 or '7, the

value of 11$ for an unknown particle energy value the plot is entered on the-abscissa corresponding to theatomic weight of the material of the target, line 25', and the plot is entered on the ordinate corresponding to the observed value of ds, line 26,]and' thevalue of the primary particle energy determined.

- 'There'isrthus disclosed a unique-method of'and apparatus for determining the values of the'sec ondary emission ratio for known radioactive radiation from which datamay be obtained'to determine the values of unknown radioactive radiations -What is claimed is:

1.- The method of determining the unknown energy of nuclear charged particle radiation com- I 1 prising: determining the secondary emission ratio for known energy particle radiation bombarding targets of known material, determining thesecondary emission-energy characteristics of said known targets, bombarding a target of known material by the saidunknown energy radiation and determining the. secondary emission ratio thereof, and'applying said last mentioned ratioto saidcharacteristics, whereby the corresponding energy of said particles of unknown energy is determined.

"2; The method of determining the unknown energy of nuclear charged particle radiation comprising: determining the secondary emission ratio for known energy particle' radiation bombarding targets of known elements, plotting said determinations in curves having as one variable the secondary emission ratio and as the other variable the-particle energy, bombarding a target of known material'by the said unknown energy radiation and :determining the secondary emission ratio thereof, and applying said last mentioned ratio to said curves, whereby the corresponding energy of the said particle of unknown energy is determined.-

3." The method of determining the unknown energy of nuclear charged particle radiation comprising: determining the secondary emission ratio for known-energy particle radiation bombarding targets of known compounds of elements, plotting said "determinations in curves having as one variable thesecondary'emission' ratio and as the other variable the particle energy, bombarding a target of known materialby the said unknown energy radiation and determining the secondary emission ratio thereof, and applying said last mentioned ratio to said curves, whereby the corresponding energy of said particles of unknown energy is determined.

"4. The method of determining the unknown energy ofnuclear charged particle radiation-comprisingidetermining the secondary emission ratio for known energy particle radiaticn bombarding targets of known compounds of elements, plotting said determinations in curves having as one variablethe secondary emission ratio and as'the other variable the particle energy, bombardinga plurality of saidtargets of known secondary emission characteristics by the said unknown energy radiationand determining thesecondary emission ratios-thereof, and applying the differences --be- 7 tween said determined ratios to said curves, Whereby the corresponding energy of the said particles of unknown energy is determined.

- 5. The method of determining the unknown energy of nuclear charged particle radiation comprising: determining the secondary emission ratio for known energy particle radiation bombarding targets of known elements, plotting said determinations in curves having as one variable the secondary emission ratio and as the other variable the particle energy, bombarding a plurality of said targets of known secondary emission characteristics by the said unknown energy radiation and determining the secondary emission ratios thereof, and applying the differences between said determined ratios to said curves, whereby the corresponding energy of the said particles of u.n--v known energy is determined.

6. The method of determining the ratio of secondary electron emission of a target exposed to primary charged particle radiation comprising: collecting the said secondary electrons on an electrode and conducting them to a point, conductin the free electrons on said target to said point, and grounding said point, the said ratio being the value of said collected current divided by the value of said ground current.

'7. The method of determining the ratio of secondary electron emission of a target exposed to primary charged particle radiation comprising: collecting the said secondary electrons on an electrode and conducting them to a point, biasing said electrode with respect to said target, conducting the free electrons on said target to said point, and grounding said point, the said ratio being the value of said collected current divided by the value of said ground current.

8. In a radioactive charged particle radiation energy meter including a target of two sections of dissimilar materials and a collector adjacent. thereto, the method of determining theunknown charged particle energy of said radiation comprising: determining the secondary emission ratios for known nuclear particle energy radiation for the two target materials, plotting said determinations in curves having as one variable the secondary emission ratios and as the other variable the particle energy, bombarding the said target by the unknown energy radiation and determining the difference between the secondary emission ratios of the two sections of the target, and applying the said determined difference to said curves, whereby the corresponding energy of the particles of unknown energy is determined.

9. In an electronic device including a source of nuclear charged particle radiation, a secondary emission responsive target bombarded by said radiation, the said target consisting of a pair of separated semicircular sections a and b of different known secondary emission responsive characteristics and a collector for said emission, the method of determining the energy of said radiation comprising: bombarding said sections by said radiation, collecting said secondary emission currents from said targets separately and conducting them separately to a point, conducting the free electron currents from said sections separately to said point, grounding said point, thereby conducting currents from said point to ground (ipa plus ipb) biasing the said conductor with increasing negative potential until the said collecting currents are zero, whereby the currents from said points to said sections are reduced respectively to the primary currents flowing to said sections (ipa and ipb), the difference betweenthe secondary emission ratio being defined by the equation dsa dsb equals g zpa zpb and applying said difference to said characteristics whereby the energy of the particles of said radiation is determined; I

10. In an electronic device including a source of nuclear charged particle radiation, a secondary emission responsive target bombarded by said radiation, the said target consisting of a pair of separated semicircular sections a and b of different secondary emission responsive materials and a collector for said emission, the method of determining the difierence between the secondary emission ratios of said sections (dsa and dsb) bombarded by a radiation of unknown energy, comprising: collecting said secondary emission currents 'from said targets (isa and isb) and conducting them separately to a point, conducting the free electron currents from said target sections separately to said point, grounding said point thereby conducting currents from said point to ground (ipa plus z'pb), biasing the said collector with increasing negative potential until the said collecting currents are zero, whereby the currents from said points to said sections are reduced respectively to the primary currents flowing to said sections (z'pa and ipb), the difference between the secondary emission ratios being defined by the equation:

11. In an electronic device including a source of nuclear charged particle radiation, a secondary emission responsive target bombarded by said radiation, the said target consisting of a plurality of alternately positioned strips of different known secondary emission responsive characteristics and a collector for said emission, the method of determining the energy of the particles or" said radiation comprising: collecting separately the said secondary emission currents 512 and z'sb and conducting them to a point, conducting the free electron currents from said target sections separately to said point, grounding said point thereby conducting currents from said point to ground (ip), the difference between said secondary emission ratio being defined bythe equation:

dsa-dsb equals i -M zp zp and applying said difference to said characteristics whereby the energy of said radiation is determined.

12. In an electronic device including a source of nuclear charged particle radiation, a secondary emission responsive target bombarded by said radiation, the said target consisting of a plurality of alternately positioned strips of different known secondary emission responsive characteristics, and a collector for said emission, the method of determining the difference between the secondary emission ratios of said sections (dsa and dsb) bombarded by a radiation of unknown energy, comprising: collecting separately the said secondary emission currents z'sa and is?) and conducting them to a point, conducting the free electron currents from said target sections separately to said point, grounding said point thereby conducting currents from said point to ground (ip) the dif said responsive means consisting of two semicircular discs of different secondary emission re sponsitivity, a collector positioned adjacent said responsive means, means for connecting each 01' tron em'jijssion responsive target bombarded by i said radiation, and a collector for said secondary electronieimission, the method of determining the ratio of sam secondary emission currents to said primaryi radiation currents comprising: conducting currents from said collector and currents from said target to a grounded common point, determining t'jhe currents flowing from said collector to said point and from said point to said ground, the said ratio being the value of said collector currents divided by the value of said ground currents.

14. In"; combination: a source of radioactive charged particle radiation, a secondary emission responsive means exposed to said radiation, means for collecting said emission, the said emission means and said collecting means being connected-together at a point and the said point being f grounded, and means for measuring the currentsi'flowing to and from said point.

15. The combination defined by claim 14 characterizedby the collection means being a hollow truncated cone positioned adjacent said re:

sponsivef fmeans, the axis of said cone being at right angles to the plane of said responsive means and at the center thereof.

16. combination: a source of radioactive chargedl'particle radiation, a secondary emissionresponsive means exposed to said radiation, the

7 said discs to a point, means including a biasing electric source connecting said collector to said point, means including a current measuring device connecting said point to ground, and means for measuring respectively the currents flowing between said point and said discs.

17. In combination: a source of radioactive charged particle radiation, a secondary emission responsive means exposed to said radiation, the said responsive means consisting of two sets of strips of difierent secondary emission ratio re- I sponsitivity positioned alternately across the said radiation, the'alternate strips being connected together, means for connecting respectively each said set of strips to a point, means including a REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Date Number Name 2,205,055 Zworykin et al. June 18, 1940 2,227,103 Orthuber et a1 Dec. 31, 1940 2,288,256 Shepherd June 30, 1942 

